This invention relates to apparatus for signalling within a borehole while drilling, and is more particularly concerned with a down-hole signal transmitter for a mud-pulse telemetry system.
Various types of measurements-while-drilling (MWD) systems have been proposed for taking measurements within a borehole while drilling is in progress and for transmitting the measurement data to the surface. However to date only one type of system has enjoyed commercial success, that is the so-called mud-pulse telemetry system. In that system the mud stream, which passes down the drill string to the drill bit and then back up the annular space between the drill string and the bore wall with the object of lubricating the drill string and carrying away the drilling products, is used to transmit the measurement data from a down-hole measuring instrument to a receiver and data processor at the surface. This is achieved by modulating the mud pressure in the vicinity of the measuring instrument under control of the electrical output signal from the measuring instrument, and sensing the resultant mud pulses at the surface by means of a pressure tranducer.
The applicants' British Patent Specifications Nos. 2,082,653A and 2,087,951 disclose such a system in which a flow constrictor defines a throttle orifice for the mud passing along the drill string, and a throttling member is displaceable under control of the electrical output signal from the measuring instrument to vary the throughflow cross-section of the throttle orifice and to thereby modulate the mud pressure. The system includes a turbogenerator driven by the mud flow for supplying electrical power to the measuring instrument.
It is an object of the invention to provide a generally improved down-hole signal transmitter which is particularly compact and well adapted to operation down-hole in a hostile environment.